Microwave antennas may emit energy having a radiation pattern that includes a main lobe and side lobes. The side lobe energy may be undesirable. For example, the side lobe energy may draw energy from the main lobe and may make detection of an emitter easier. Side lobe energy may be reduced by reducing longitudinal edge currents at the mouth of an aperture antenna or a waveguide. The longitudinal edge currents may be reduced by propagating energy in a mixed propagation mode including a dominant propagation mode and a higher order propagation mode to cancel longitudinal current. The mixed propagation mode may result from converting energy propagating in the dominant propagation mode to energy propagating in the higher order propagation mode. A dimension (e.g., a cross-sectional area of an interior region) of the waveguide may be varied along its length in order to present a boundary value perturbation that causes energy propagating in the dominant propagation mode to convert to energy propagating in the higher order propagation mode. For example, the wall of the waveguide may include a flare, an iris, a groove, or a step to convert energy to the higher order propagation mode. However, varying the cross-sectional area of the waveguide wall may be undesirable. For example, many systems include waveguides that have a substantially constant cross-sectional area and it would be costly to replace the waveguides in these systems.